It is known to control the brightness of a lamp in a lamp unit connected to AC mains voltage by contacting an electrically conducting part of the lamp unit or lamp that serves as a sensor. Such a control is known, for example, from DE magazine "Funk-Technik", Vol. 37, No. 5, 1982, page 192.
In such a known circuit, the luminous means of a lamp is switched on by a first, brief touch on the electrically conducting part of the lamp serving as sensor. By prolonged touching of the sensor, the brightness of the luminous means can be altered, if required. Switching off of the luminous means is effected by another short touch on the sensor. This known circuit arrangement comprises a luminous means that is connected to AC mains voltage via a triac. A control unit having the sensor connected thereto issues a control signal to the triac. The control signal turns the triac on or off or controls the same in the conducting state in accordance with the desired brightness of the lamp. The brightness control of the lamp is effected in accordance with a phase angle control which is determined by the instantaneous value of the control signal.
In the control unit disclosed in the above-mentioned document, touching of a so-called touch area by a user causes a change in amplitude of the mains voltage stepped down by means of a high-impedance voltage divider. By sensing such an amplitude change, the lamp can both be switched on and off, or the brightness thereof can be controlled.
Moreover, there are circuit arrangements known in which the control unit contains an oscillator circuit that is powered by a direct voltage obtained from the mains voltage by a rectifying and conversion circuit. An output of the oscillator circuit is coupled to the electrically conducting part of the lamp serving as sensor. When a user touches the electrically conducting part of the lamp serving as sensor, the load of the oscillator contained in the oscillator circuit changes, thereby changing the frequency and/or the amplitude of the output signal of the oscillator. This change of the output signal is sensed by an evaluation circuit that is also contained in the control circuit. In accordance with such a change, the control signal for controlling the triac is produced.
Circuit arrangements of the type described hereinbefore, however, cannot be employed when the lamp to be operated has to fulfill the requirements of protection class 1 of the VDE guidelines. The requirements of protection class 1 prescribe that the phase conductor or the neutral conductor of an electric appliance must be insulated in simple manner from housing parts of the appliance. Furthermore, electrically conductive parts of the housing must be connected in electrically conducting manner to a protective or earth conductor at ground potential. Such systems for the power supply of electric appliances, using three conductors, namely a phase, a neutral conductor and a protective conductor, are frequently used. However, by using a protective conductor connected to electrically conductive parts e.g. of a lamp housing, it is not possible to use electrically conductive parts of the lamp housing as sensor, as described hereinbefore for a lamp the electrically conductive housing parts of which are not connected to a protective conductor.
The circuit arrangements of the type described hereinbefore thus can be used only for lamps fulfilling the requirements of protection class 2 of the VDE guidelines, which require no protective conductor. This presents a disadvantage to the effect that lamps fulfilling the requirements of protection class 2 involve much more expenditure in construction and manufacture as compared to lamps fulfilling the requirements of protection class 1. U.S. Pat. No. 4,701,676 already reveals a circuit arrangement for energizing and de-energizing and for dimming a lamp containing an oscillator circuit.